An incredible abundance of elemental sulfur, nearly 7-million tons is generated as a waste byproduct from hydrodesulfurization of crude petroleum feedstocks, which converts alkanethiols and other (organo) sulfur compounds into S8 as described in Chung and which is hereby incorporated herein by reference in its entirety. Before the invention of the inverse vulcanization process, there were only a limited number of synthetic methods available to utilize and modify elemental sulfur. Current industrial utilization of elemental sulfur is centered around sulfuric acid, agrochemicals, and vulcanization of rubber. For example, elemental sulfur is used primarily for sulfuric acid and ammonium phosphate fertilizers, where the rest of the excess sulfur is stored as megaton-sized, above ground sulfur towers.
While sulfur feedstocks are plentiful, sulfur is difficult to process. In its original form, elemental sulfur consists of a cyclic molecule having the chemical formulation S8. Elemental sulfur is a brittle, intractable, crystalline solid having poor solid state mechanical properties, poor solution processing characteristics, and there is a limited slate of synthetic methodologies developed for it. Hence, there is a need for the production of new materials that offers significant environmental and public health benefits to mitigate the storage of excess sulfur in powder, or brick form.
Elemental sulfur has been explored for use in lithium-sulfur electrochemical cells. Sulfur can oxidize lithium when configured appropriately in an electrochemical cell, and is known to be a very high energy-density cathode material. The poor electrical and electrochemical properties of pure elemental sulfur, such as low cycle stability and poor conductivity) have limited the development of this technology. For example, one key limitation of lithium-sulfur technology is the ability to retain high charge capacity for extended numbers of charge-discharge cycles (“cycle lifetimes”). Cells based on present lithium ion technology has low capacity (180 mAh/g) but can be cycled for 500-1000 cycles. Lithium-sulfur cells based on elemental sulfur have very high initial charge capacity (in excess of 1200 mAh/g, but their capacity drops to below 400 mAh/g within the first 100-500 cycles. Hence, the creation of novel copolymer materials from elemental sulfur feedstocks would be tremendously beneficial in improving sustainability and energy practices. In particular, improved battery technology and materials that can extend cycle lifetimes while retaining reasonable charge capacity will significantly impact the energy and transportation sectors and further mitigate US dependence on fossil fuels.
There have been several recent attempts to form sulfur into nanomaterials for use as cathodes in lithium-sulfur electrochemical cells, such as impregnation into mesoporous carbon materials, encapsulation with graphenes, encapsulation into carbon spheres, and encapsulation into conjugated polymer spheres. While these examples demonstrate that the encapsulation of elemental sulfur with a conductive colloidal shell in a core/shell colloid can enhance electrochemical stability, these synthetic methods are challenging to implement to larger scale production required for industrial application. Hence, a new family of inexpensive, functional materials obtained by practical methods is desirable.
Polymeric materials and structures commonly experience physical damage or structure failure due to factors affecting their physical properties, such as environmental exposure or mechanical or thermal stress. According to Colquhoun, which is hereby incorporated herein by reference in its entirety, there is a growing interest in developing new polymers that have the ability to repair themselves in order to enhance and prolong the life of these polymers. However, these new polymers require external energy or agents for repair. Hence, there is a desire to develop polymeric materials capable of self-healing that also require little to no external intervention.
Elemental sulfur is inherently insulating and poorly photoactive. One key challenge is to understand the synthetic chemistry necessary for modification of sulfur to prepare photoactive materials, especially materials that act as photo-semiconductors. However, by utilizing the inverse vulcanization process to enable the synthesis of modified sulfur polymers, the waste sulfur can be transformed from an insulator into a photoelectrochemically active material.
Aside from using sulfur to form nanomaterials for use in a variety of applications, such as cathodes in lithium-sulfur electrochemical cells, optics and weapon production, another particular field of interest is the use of sulfur polymer nanoparticles as hydrogen sulfide (H2S) donors. According to Hasegawa, which is hereby incorporated herein by reference in its entirety, H2S has been recognized as a third gaseous transmitter produced endogenously from cysteine and other enzymes for signaling purposes. Nitric oxide (NO) and carbon monoxide (CO) are also examples of signaling gases. Although considered a toxic compound, H2S gas has beneficial effects on human organs and biological processes at low concentrations. Examples of these benefits include cardioprotection, inflammation regulation, cell protection during ischemia reperfusion and neuro-inflammation, and vasodilation, as described in Foster and which is hereby incorporated herein by reference in its entirety. Furthermore, unlike NO and CO, H2S gas does not emit reactive oxygen species that could affect certain cell functions. Limited studies have been performed on H2S as a signaling gas and identifying potential H2S donors. Previous studies utilized salts such as NaHS/Na2S, which have demonstrated uncontrolled release profiles (Hasegawa). Hence, there is a need to develop H2S donors capable of slowly and continuously releasing H2S gas.
Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one of ordinary skill in the art. Additional advantages and aspects of the present invention are apparent in the following detailed description and claims.